Related Field
The present invention relates to an electric submersible pump system. In particular, the present invention relates to a power supply for use with an electric submersible pump system. The power supply can be used to power down-hole electronics, for example down-hole communication systems.
FIG. 1 shows a typical oil-well electric submersible pump (ESP) system. This has a down-hole three phase cable, a three phase ESP motor, an ESP seal section (an oil reservoir and bearing assembly), an ESP pump and a remote electronic instrumentation system with power-line communications capability. FIG. 2 shows a single-line diagram for the system of FIG. 1. This has a 480V AC supply, a multi-tapped, step-up isolating transformer, a three phase cable and a medium voltage motor. The 480V AC supply may be fixed or variable frequency. FIG. 3 is a simplified electrical schematic of an isolated portion of FIG. 2. This shows the inductance of the step-up transformer's secondary winding, the resistance of the cable and the inductance of the medium voltage motor. The step-up transformer and motor are both shown wye-connected as is typical.
Description of Related Art
Power-line communication systems for electric submersible pumps configured as shown in FIGS. 1 to 3 are well known. For example, U.S. Pat. No. 3,340,500 describes a system for translating electrical energy and information represented by a varying electrical signal between a surface location at the top of a deep well or shaft over a cable to the bottom of such a shaft. A sensing and indicating circuit is completed by the ground connections at the down-hole and surface locations. U.S. Pat. No. 5,539,375 describes down-hole instrumentation that includes active electronic components to provide a sequence of signals. WO 2004/028064 describes a method for communicating messages between down-hole equipment and surface equipment.
Power-line communication systems are typically direct current, loop-powered, milliamp transmitters which are connected to the two system neutrals using simple inductor-capacitor filters. FIG. 4 shows a typical state-of-the-art ground-loop power-line communications system connected to a typical isolated ESP power system. This has a DC current loop source at the surface, and a remote, downwell DC current loop sink. Typical values for the DC resistances of the various conductors and windings are shown. For example, using the DC current loop source a DC voltage of 36V may be applied to the step-up transformer's neutral wire via a first inductor-capacitor filter. The resistance (ZL1) of the inductor may be 600 ohms. The current supplied to the system (Isource) is measured. This current may be decoded as the sum of the remote device's power supply current and information-carrying signal current.
At the ESP motor a single remote loop-powered transmitter is connected to the motor neutral using a second inductor-capacitor filter. A typical resistance for inductors of the type used is 600 ohm. The remote transmitter incorporates a power supply unit (drawing for example 10 mA) and a variable resistance. By varying resistance over time information can be transferred. The variable resistance is typically implemented as an electronic circuit which modulates the total current draw (Isink) between the power supply current (typically 10 mA) and an upper value (typically 18 mA). This modulation may carry analog or digital information.
In use, when the modulation is at zero-scale the only current draw is the power supply's 10 mA current draw. The voltage drop through the inductors is 6V across each making 12V drop in total. In this condition 12 VDC is dropped across ZL1 and ZL2 and 24 VDC reaches the remote device's power supply (Vpsu). With the modulation at full-scale (18 mA) the drop across ZL1 and ZL2 is (2*600*0.018=21.6V). At this full-scale modulation Vpsu drops to 14.4 VDC.
A limitation of known power-line communication systems is that they have a low power transfer capability. In the example above, the power available at the power supply unit of the remote device is (14.4 VDC*10 mA=0.144 W). Reducing the inductor resistance, increasing the voltage supplied to around 180 VDC and reducing the modulation range have been employed as strategies to increase power transfer capability by 1-2 W.
Another limitation is the adverse effect of system grounding. FIG. 5 shows a fault at the bottom of the phase A winding of the motor. The fault resistance is shown as 0 ohm, typical of what may happen should fluid from the oil well leak into the ESP motor and collect at its base. This presents a short circuit across the remote inductor-capacitor filter and it becomes impossible to transfer power to the remote device. A typical minimum Vpsu required to operate the remote device may be 8 VDC. In the described system the minimum Vpsu threshold is breached should the fault have a resistance of 3,300 ohms or lower.
Yet another limitation is the bandwidth achievable for data communications. The inductor-capacitor filter of these systems is designed to pass DC (and very low frequencies) and reject power frequencies above 20-30 Hz. As a result, they do not have the capacity to pass high frequency signals and the bandwidth is limited to around 5-10 bits-per-second.
A further limitation is that it is difficult to deploy more than one loop-powered transmitter in parallel within the system. This is because if one transmitter fails by shorting to ground then all the transmitters will fail. Also if one transmitter fails by not maintaining a stable current when it is not communicating then this affects all the transmitters. Furthermore, a time-division-multiplex scheme needs implemented between the multiple transmitters and this reduces the bandwidth per device from what is already a low capacity. Also, as each system is operated in parallel each draws a power supply current and this increases the total current. This becomes impractical due to the need to pass these multiple currents through the inductor ZL1.
AC based systems have also been developed. Recent examples of AC communications systems are described in WO2013/132231, WO2013/132232, WO2013/132233 and WO2013/132234. In AC based systems, a down-hole unit is AC coupled to conductors of a power cable through a wye point of an ESP motor assembly and a surface unit is AC coupled to conductors of a power cable.